The invention relates to transporting a load by means of load handling equipment and particularly to measurements in load handling equipment in connection with transporting a load.
Today, a vast majority of international sea freight is transported in containers. Containers are boxlike transport units having standard dimensions, either 20, 40 or 45 feet in length. A container is about 2.5 meters in width, and the most typical container heights are about 2.6 meters and 2.9 meters.
Containers are used in land and sea transport between countries and continents. Container dimensions are standardized, whereby it is easier to handle them by various vehicles. In ports, containers are handled by cranes, such as gantry cranes. Within a port area, containers are moved from one place to another or from one vehicle to another, for example from a ship to a port area and from a port area to a vehicle used in internal or external traffic of the port area. Vehicles used in internal and external traffic of a port area include trucks, carriages and railway carriages, for instance. Carriages used in internal traffic of a port area may be highly automated.
Containers are provided with standardized corner castings enabling a container to be hoisted and carried by different container cranes. A container crane is typically provided with a spreader suspended on hoisting ropes or chains, the spreader being adjusted by a telescopic mechanism according to the length of the container to be picked up, for example to a length of 20 or 40 feet. The corners of a spreader are provided with special turnable twist-locks enabling a container to be gripped. The corner castings of the container are provided with standardly shaped holes in which the twist-locks of the spreader are fitted. When the container crane lowers the spreader on top of a container such that all four twist-locks of the spreader are received in the holes of the corner castings, the twist-locks may subsequently be turned by 90 degrees, making the twist-locks lock into the corner castings. The container may then be lifted into the air, suspended from the spreader.
Containers may be stacked on top of one another, typically for instance five containers on top of each other. This enables a large number of containers to be stored within a small area, such as in a container port. The stacking of containers has to be carried out carefully such that the corner castings at the bottom of the container to be stacked are aligned with the corner castings on the roof of the lower container with an accuracy of at least about 5 cm. Otherwise there is a risk of the container stack collapsing.
A typical container crane used for picking up and stacking containers is called a gantry crane, which may move either on rails (RMG, Rail Mounted Gantry Crane) or on rubber tyres (RTG, Rubber Tyred Gantry Crane). When a gantry crane is used, containers are handled between the legs of the gantry crane. Typically, a driveway is left between the legs of the gantry crane to enable containers to be driven along it underneath the gantry crane to be stacked into rows or to be moved to another vehicle.
Laser scanners may be mounted on fixed posts in the port area to facilitate the handling of containers. The position data obtained from the laser scanners is presented in a coordinate system connected to a post. However, the coordinate system used for controlling the crane is typically a coordinate system different from that of the laser scanner, positioned on a post. Thus, utilizing the position data from the laser scanners in handling containers requires matching a plurality of coordinate systems. For the matching, the measurement results obtained from the different coordinate systems must be calibrated relative to each other. Calibrating may, however, cause errors, which deteriorates the accuracy of container handling. If a laser scanner fixedly connected to a geographic coordinate system is utilized by several cranes, all cranes should, in practice, be given similar crane coordinate systems at a given accuracy. In rubber-tyred cranes, for example, tyre pressures that change constitute one possible time-varying error. Errors in the accuracy of container handling may lead to control errors in container handling and cause a danger to personnel and equipment.
Calibrating may have to be redone or at least checked at even intervals, for instance in connection with maintenance operations or installments, and then the crane cannot be used. This results in a decrease in the efficiency of crane operation, which may affect cranes operated in the same area, such as in a port, and the efficiency of operation in the entire port area.